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Keywords:

  • Childhood obesity;
  • school performance;
  • self-esteem

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References

Consequences of obesity for mental health and cognitive development are not established to the same degree as those for chronic diseases. This study aims to document the interrelationships between body weight, self-esteem and school performance in childhood. Height and weight measurements and self-report of self-esteem, diet quality and physical activity of 4945 grade 5 students were linked with standardized literacy test results. Structural equation models were applied to confirm hypothesized relationships between body weight, self-esteem and school performance, and revealed that body weight affected self-esteem negatively and that school performance affected self-esteem positively. Body weight did not affect school performance, and self-esteem did affect neither body weight nor school performance. Subsequent multi-level logistic regression showed that obese students, relative to normal weight students, were more likely (1.44; 95% CI: 1.12–1.84), and students with good school performance, relative to those performing poor, were less likely (0.39; 95% CI: 0.26–0.58), to have low self-esteem. Diet quality and active living had positive effects on both school performance and self-esteem. The study findings further establish obesity as a risk factor for low self-esteem and add to the rationale to promote healthy eating and active living among children and youth as this will prevent chronic diseases and improve mental health and cognitive development.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References

Childhood obesity is a major public health problem as it increases the risk for various chronic diseases resulting in decreases in life expectancy and quality of life (1,2). The importance of childhood obesity for mental health and cognitive development has been less studied. Self-esteem is one of the prominent components of mental health and personality development (3,4). Low self-esteem may hamper social and cognitive development in children and may lead to poor health outcomes in adulthood (5). Poor cognitive development may result in limited educational attainment, impaired career opportunities and in the long-term lower socioeconomic status with reduced prospects for good health (6,7).

Childhood obesity rates have dramatically increased in the past decades (8). In Canada in 2004, about 26% of children were overweight or obese, and 8% of children were obese (9). If childhood obesity rates continue to increase and if childhood obesity has a negative effect on self-esteem and cognitive development, one should expect more mental health problems and less educational attainment adding to the health burden in adulthood and that of future generations (10).

Previous studies have suggested a modest inverse association between body weight and self-esteem (11,12), a positive association between school performance and self-esteem (13,14), and a possible association between obesity and school performance (15). It may be beneficial to study body weight, self-esteem and school performance simultaneously, as studies have suggested that they are interrelated. To date, however, only one earlier study did consider body weight, self-esteem and school performance simultaneously (14). This study reported that higher body mass index (BMI) was associated with lower self-esteem and with poorer grades among high school students (14). No earlier study has reported on the interrelationships of obesity, self-esteem and school performance among younger children.

In the present study, we aim to reveal the relationships between obesity, self-esteem and school performance among grade 5 students who are primarily 10 and 11 years old.

Research methods and procedures

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References

The survey

The 2003 Children's Lifestyle and School-Performance Study (CLASS) is a large study of fifth grade students and their parents in the Canadian province of Nova Scotia, where more than 95% of the population is of European descent and 98.4% of students attend public school (16). The study targeted fifth grade students as these children would also complete a standardized school performance test. Of the 291 public schools with grade 5 classes, 282 (96.9%) participated by distributing a consent form and questionnaire to the parents of all their fifth grade students. Parental consent was received for 5517 students, resulting in an average response rate of 51.1% per school. Trained CLASS representatives visited these schools during school hours to administer the Harvard's Youth Adolescent Food Frequency Questionnaire (YAQ) (17), a student survey with questions on self-esteem and physical and sedentary activities, and to measure the heights and weights of participating students. The administration of the surveys and measurement of heights and weights took generally less than 45 min to complete. Further details on the conduct of the CLASS study are provided elsewhere (18).

Height and weight

After removing their shoes, CLASS representatives measured standing height to the nearest 0.1 cm and body weight was measured to the nearest 0.1 kg using calibrated digital scales (18). Overweight and obesity were defined by using the international BMI cut-off points established for children and youth by the International Obese Task Force (19). These cut-offs are based on health-related adult definitions of overweight (≥25 kg m−2) and obesity (≥30 kg m−2), but are adjusted to specific age and gender categories for children.

School performance

The Elementary Literacy Assessment is a standardized test administered by the Nova Scotia Department of Education. Completion of the assessment required students to read a variety of materials and answer written questions based on those readings. Materials included a short story, information texts, a poem and a visual media text. Reading and writing assessments were marked centrally by a team of experienced teachers under the supervision of the Nova Scotia Department of Education. Both individual and school-level test results were linked to the CLASS database. The available school performance for reading and for writing included an indicator of ‘not meeting the standard’, ‘meeting the standard’ or ‘exceeding the standard’. At the school level, we considered the percentage of students meeting or exceeding the standard for both reading and writing as a marker of performance.

Self-esteem

A large number of instruments have been developed in the past decades to assess self-esteem in distinct groups and for distinct purposes (20,21). From these existing instruments, we selected 11 items suitable for our age group to capture appearance, abilities, emotions, peer acceptance, future hopes and overall life satisfaction levels in a population-based setting. Specifically, we included the following items (i) I like myself; (ii) I feel like I do not have any friends; (iii) I like the way I look; (iv) My future looks good to me; (v) I feel unhappy or sad; (vi) I worry a lot; (vii) I cry a lot; (viii) I get into physical fights; (ix) I am bullied by other kids; (x) I bully other kids and (xi) I have trouble paying attention. Response choices for each of these items included ‘never or almost never’, ‘sometimes’ to ‘often or almost always’. We scored these responses as 1, 2 and 3, with the highest score representing good self-esteem, and subsequently totalled the scores (20). The self-esteem score therefore ranges from 11 to 33. The inter-item reliability (Cronbach's alpha) of the 11 items was 0.78.

Other influencing factors

The interrelations between body-weight status, self-esteem and school performance are affected by various factors. We considered the following: child's gender, rural or urban residency, household income, parental education, child's physical and sedentary activity level, diet quality, school neighbourhood income level and school-level academic performance. Age was not considered, as all participants were fifth graders who are primarily 10 and 11 s old. The questionnaire completed by parents provided information on location of residence, household income in four categories (less than CA$20 000, $20 000–$39 999, $40 000–$59 999 and $60 000 or more), and parental education (secondary school or less, community college, university and graduate university). The student survey included validated questions on physical activity and sedentary activity taken from the National Longitudinal Survey for Children and Youth (21). The YAQ is a 147-item food frequency questionnaire modified for use in Canada by using product names that are common in Canada (17). Information obtained through the YAQ and Canadian food tables allows estimation of food and nutrient intakes that in turn are used to calculate the Diet Quality Index-International, a composite measure of diet quality (22). School neighbourhood income level was determined by averaging the postal code level information on household income, available through Census Canada, of students attending the particular school (23).

Analytic approach

Structural equation models (SEM) are based on covariance matrices rather than individual observations. Regardless of whether observations are cross-sectional or longitudinal, SEM reveal effects and relationships, although there is some divide among researchers using SEM whether these effects and relationships are causal or not. Importantly, unlike regression methods, SEM allow studying models with more than one outcome variable, such as in our study, where we a priori consider three outcome variables: body weight, self-esteem and school performance. As a first step in our analytic approach, we applied SEM to determine the relationships between body weight, self-esteem and school performance (the three endogenous concepts). We started with trying to confirm a model that included each of the six relational combinations (i) body weight affects self-esteem; (ii) self-esteem affects body weight; (iii) body weight affects school performance; (iv) school performance affects body weight; (v) school performance affects self-esteem and (vi) self-esteem affects school performance. Upon failure to confirm this model, i.e. when the goodness-of-fit χ2 test statistic revealed a statistically significant difference between the hypothesized model and the observations, we reduced the number of relational combinations by removing the relational combination with the poorest fit. Subsequently, we tried to confirm the reduced model. We repeated this process until we confirmed the existence of a model with a satisfactory goodness of fit (P > 0.05). In the above models, we considered the confounding influence of child's gender, rural or urban residency, household income, parental education, child's physical and sedentary activity level, diet quality, school neighbourhood level income, and school-level academic performance (the exogenous concepts) on body weight, self-esteem and school performance.

The application of SEM comes with established limitations (24). For example, endogenous concepts are preferably continuous or ordinal with multiple categories (24). For this reason, we considered BMI as a continuous concept of body weight, self-esteem score as a continuous concept and school performance as an ordinal concept of five categories. Details are provided in Table 1. A second limitation is that SEM do not easily accommodate clustering of observations of students within that of their schools.

Table 1.  Variables and their coding in structural equation models and multilevel regression analysis
  Structural equation modelsMultilevel regression
  1. BMI, body mass index.

Body-weight statusBMI from measured height and weightWeight (kg)/height (m2) (considered continuous)1 = normal weight
2 = overweight
3 = obesity
School performanceProvince test on reading and writing1 = both reading and writing below standard1 = below standard (reading and/or writing below standard)
2 = one below, one meets standard2 = meets standard (neither category 1 nor 3)
3 = both meet standard or one below and one exceeds standard3 = exceeds standard (exceed standard of both reading and writing)
4 = one meets, one exceeds standard
5 = both exceed standard
Self-esteemSelf-reportScore ranges from 0 to 330 = normal self-esteem
1 = low self-esteem
GenderSelf-report1 = girl, 2 = boy1 = girl, 2 = boy
ResidenceParent report1 = rural, 2 = urban1 = rural, 2 = urban
Household incomeParent report1 = $20 000 or less1 = $20 000 or less
2 = $20 001–$40 0002 = $20 001–$40 000
3 = $40 001–$60 0003 = $40 001–$60 000
4 = $60 001 or more4 = $60 001 or more
Parental educationParent report1 = secondary or less1 = secondary or less
2 = community college2 = community college
3 = university3 = university
4 = graduate university4 = graduate university
Physical activitySelf-report1: ≤twice week−11: ≤twice week−1
2: >2 and ≤4 times week−12: >2 and ≤4 times week−1
3: >4 and ≤7 times week−13: >4 and ≤7 times week−1
4: >7 times week−14: >7 times week−1
Sedentary activitySelf-report1: ≤1 h d−11: ≤1 h d−1
2: >1 and ≤3 h d−12: >1 and ≤3 h d−1
3: >3 and ≤6 h d−13: >3 and ≤6 h d−1
4: >6 h d−14: >6 h d−1
Diet Quality IndexCalculated from Harvard Food Frequency QuestionnaireScore ranges from 0 to 100 (considered continuous)1 = 1st tertile (lowest)
2 = 2nd tertile (average)
3 = 3rd tertile (highest)
School neighbourhood level incomeCanada censusIn thousands of dollars (considered continuous)1 = 1st tertile (lowest)
2 = 2nd tertile (average)
3 = 3rd tertile (highest)
Considered as second level covariate
School-level academic performanceProvincial standard reading and writing testPercentage of students failing both assessments1 ≤ 10% failure
2 = 10–19% failure
3 = 20–29% failure
Considered as second level covariate

Given the above limitations and because the results of the SEM justified the use of regression methods (see result section), we applied multi-level regression methods to more accurately quantify the associations of body-weight status and of school performance with self-esteem. Specifically, we applied two-level logistic regression methods with self-esteem as the first-level binary outcome. As the responses to the self-esteem items were ordinal and the sum scores were not normally distributed, we dichotomized the outcome and defined scores lower than the 15th percentile as low self-esteem, which is close to the parametric concept of one standard deviation below the mean and which is commonly applied in self-esteem research (25). In terms of body weight, children were categorized as normal weight, overweight or obese (Table 1). Body-weight status and all other child and parent characteristics were considered as first-level covariates. School neighbourhood level income and school-level academic performances were considered as second-level covariates (Table 1). Both unadjusted results of a univariate model and adjusted results of a parsimonious multivariate model are presented. These analyses included 4945 children that had complete information of self-esteem and diet quality. We excluded 234 (4.5%) of the 5200 students with outlying observations based on energy intakes less than 500 kcal or greater than 5000 kcal per day in accordance with established recommendations for outliers in nutritional research (26). We excluded an additional 21 (0.4%) students who had incomplete information of self-esteem. Missing values for other covariates were considered as separate covariate categories.

The SEM were constructed using LISREL, version 8.0, and the remaining statistical analysis conducted with STATA, version 9.0.

Ethics approval

The Health Sciences Human Research Ethics Board of Dalhousie University has approved the original study, including the informed consent procedure. The Health Research Ethics Board at the University of Alberta has approved the data analysis of the present study.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References

Our structural equation modelling confirmed the existence of three interrelations among the three endogenous concepts, two of which were statistically significant. (i) Body weight affects self-esteem whereby an increase of one unit of BMI is associated with a decrease of 0.048 (95% confidence interval [CI]: −0.070, −0.025) in self-esteem score. (ii) School performance affects self-esteem whereby an increase of one unit of school performance is associated with an increase of 0.609 (95% CI: 0.505, 0.712) in self-esteem score. (iii) Body weight affects school performance, though not statistically significantly, whereby an increase of one unit of BMI is associated with a decrease of 0.003 (95% CI: −0.010, 0.004) in school performance. Self-esteem did affect neither BMI nor school performance. And school performance did not affect BMI.

Table 2 shows the associations (and 95% CIs) of the exogenous concepts with each of the three endogenous concepts. Figure 1 displays those statistically significant associations of exogenous concepts with endogenous concepts (γ's) and those interrelations between endogenous concepts (β's). Socioeconomic factors, both parental education and household income, affect BMI, self-esteem and school performance in a statistically significant manner. Physical activity affects self-esteem in a positive manner and sedentary activities affect self-esteem and school performance in a negative manner. Diet quality has a positive effect on school performance (Table 2 and Fig. 1). The goodness-of-fit χ2 test statistic (Table 2, P = 0.8003) indicates that the obtained model provides a strong representation of the observations.

Table 2.  Structural coefficients and 95% confidence intervals of exogenous variables with BMI, school performance and self-esteem
Exogenous variablesBMISchool performanceSelf-esteem
  • The model had a 1.0000 goodness-of-fit index (GFI), and its comparative GFI was equal to 0.9993. The goodness-of-fit chi-square was 0.4455 (d.f. = 2; P = 0.8003). Comparative Fit Index = 1.0000. Root Mean Square Error of Approximation (RMSEA) = 0.0.

  • We did not assume the existence of a direct causal relationship between school neighbourhood level income and self-esteem and between school-level academic performance and self-esteem. Models that included these relationships failed the GFI criterion, indicating a poor agreement between hypothesized model and observations.

  • BMI, body mass index.

Gender0.005 (−0.244, 0.254)−0.344 (−0.401, −0.288)−0.046 (−0.236, 0.143)
Residence−0.117 (−0.403, 0.169)0.061 (0.004, 0.126)0.283 (0.089, 0.478)
Household income−0.303 (−0.454, −0.151)0.147 (0.113, 0.182)0.217 (0.107, 0.327)
Parental education−0.429 (−0.582, −0.276)0.145 (0.110, 0.180)0.160 (0.044, 0.276)
Physical activity−0.086 (−0.248, 0.075)−0.012 (−0.048, 0.025)0.176 (0.055, 0.298)
Sedentary activity−0.010 (−0.150, 0.130)−0.040 (−0.072, −0.008)−0.429 (−0.534, −0.324)
Diet Quality Index−0.011 (−0.025, 0.003)0.006 (0.003, 0. 009)0.009 (−0.002, 0.019)
School neighbourhood level income−0.013 (−0.024,−0.002)−0.001 (−0.004, 0. 002)
School-level academic performance0.192 (0.059, 0.326)−0.103 (−0.133, 0.073)
image

Figure 1. Structural coefficients of BMI, school performance, self-esteem and exogenous variables. γ, structural coefficient between exogenous and endogenous variables. β, structural coefficient between endogenous variables. Only the statistical significant structural coefficients are displayed. BMI, body mass index.

Download figure to PowerPoint

We obtained a SEM in which body weight and school performance affect self-esteem but where no other statistically significant relations between the three endogenous concepts exist. This model can be rewritten as a model with a single outcome (self-esteem) and with body weight and school performance as independent variables. Such a model allows the application of regression methods with self-esteem as the outcome of interest. Table 3 shows the unadjusted and adjusted odds ratios for low self-esteem. Relative to normal weight participants, obese children were 1.44 times more likely to report low self-esteem. Relative to those not meeting the standard, participants of whom school performance met the standard and exceeded the standard were, respectively, 0.55 and 0.39 times less likely to report low self-esteem. These differences were statistical significant. Table 3 further shows that higher parental education, better diet quality and more physical and less sedentary activities all had a statistically significant positive effect on self-esteem.

Table 3.  Determinants of low self-esteem among grade 5 students
Independent variables% of studentUnadjusted odds ratio (95% CI*)Adjusted odds ratio (95% CI*)
  • *

    95% CI: 95% confidence interval.

  • Odds ratios are adjusted for energy intake following established recommendations (26).

  • The findings originate from 4945 students with available information on self-esteem participating in the Children's Lifestyle and School-performance Study.

  • DQI, Diet Quality Index.

Body-weight status
 Normal66.611
 Overweight23.51.18 (0.98–1.43)1.17 (0.96–1.42)
 Obesity9.91.72 (1.37–2.16)1.44 (1.12–1.84)
School performance
 Below standard (poor)18.211
 Meets standard (average)70.90.46 (0.39–0.55)0.55 (0.46–0.67)
 Exceeds standard (good)10.90.25 (0.17–0.39)0.39 (0.26–0.58)
Gender
 Girl51.01 
 Boy49.01.43 (1.23–1.66) 
Residence
 Rural34.51 
 Urban65.50.69 (0.57–0.83) 
Household income
 <$20 00011.21 
 $20 000–39 99922.40.76 (0.59–0.97) 
 $40 000–59 99926.30.58 (0.45–0.76) 
 ≥$60 00040.10.39 (0.30–0.52) 
Parental education
 Secondary school or less29.611
 Community college37.40.78 (0.66–0.93)0.86 (0.72–1.04)
 University23.60.49 (0.38–0.63)0.68 (0.52–0.88)
 Graduate university9.40.39 (0.26–0.60)0.65 (0.44–0.98)
Physical activity
 ≤twice/week12.311
 >2 and ≤4 times week–122.70.52 (0.40–0.67)0.63 (0.49–0.82)
 >4 and ≤7 times week–154.60.65 (0.53–0.79)0.74 (0.60–0.92)
 >7 times week–110.40.61 (0.45–0.83)0.78 (0.57–1.07)
Sedentary activity
 ≤1 h d−19.811
 >1 and ≤3 h d−141.00.88 (0.64–1.19)0.97 (0.70–1.34)
 >3 and ≤6 h d−126.81.23 (0.90–1.68)1.20 (0.86–1.68)
 >6 h d−122.42.13 (1.16–3.85)1.68 (1.21–2.34)
DQI
 1st tertile (lowest) 11
 2nd tertile (average) 0.53 (0.43–0.66)0.63 (0.51–0.77)
 3rd tertile (highest) 0.61 (0.51–0.74)0.81 (0.67–0.99)
School neighbourhood level income
 1st tertile (lowest) 1 
 2nd tertile (average) 0.99 (0.81–1.20) 
 3rd tertile (highest) 0.73 (0.58–0.91) 
School-level academic performance
 <10% failure41.091 
 10–19% failure19.310.92 (0.67–1.25) 
 20–29% failure12.761.22 (0.87–1.70) 
 30–39% failure26.841.20 (0.91–1.58) 

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References

Self-esteem is an assessment of one's worth and is often used as an indicator of mental well-being (3). Children and youth with low self-esteem generally have negative attitudes and limited capacity to respond to daily and developmental challenges (27,28). As such, they are more likely to engage in negative behaviours such as school absence, smoking, drinking, and drug use and delinquency, and more likely to experience negative psychological conditions such as anxiety, depression and suicidal behaviour (29,30).

This study is the first population-based study that considered obesity, self-esteem and school performance simultaneously in the young age group of 10 and 11 olds. We observed that excess body weight has an independent negative effect on self-esteem, which is in agreement with observation of studies that did not include consideration of school performance (12,31). Our observation of the positive effect of physical activity on self-esteem has also been reported previously (32). However, our observations of the positive effect of diet quality and the negative effect of sedentary activities on self-esteem have not been reported previously. They add to the rationale to promote healthy eating and active living in childhood as this will improve not only physical health, but also mental health. Consistent with our observations, various studies have shown the effects of socioeconomic factors on body weight, self-esteem and school performance (33–36). These emphasize the importance of priority for prevention initiatives targeting children of socioeconomic disadvantaged families and neighbourhoods.

The present study showed that less physical activity, more sedentary lifestyle, poor diets and excess body weight are all risk factors for low self-esteem. Children in Western countries went through decades of changes towards less physically active and more sedentary lifestyle, deteriorating diets and increased rates of excess body weight. The findings of the present study therefore suggest that as a result of these temporal lifestyle changes, the prevalence of low self-esteem may be increasing over time. Searching the existing literature, however, we were not able to confirm the existence of such a temporal increase in the prevalence of low self-esteem. To the best of our knowledge, there is no documentation on secular trends of low self-esteem in childhood. Other studies, though, did reveal that self-esteem is relatively unstable in childhood (37).

A review article published in 2005 revealed that a relative small number of studies addressed the association between childhood overweight and performance at school and that these studies have been inconsistent in their conclusions whether or not overweight children are more likely to perform poorly relative to their normal weight peers (15). The present study confirms our previous work in which we observed that poor school performance was more common among overweight children, but when considering confounding factors, the association between body weight and school performance vanished (38). A few studies made those same observations (39,40). Some of these studies had considered health behaviours like physical activity and diet quality, while others considered race and socioeconomic factors. These studies suggest that body weight is just an intermediate step and that physical activity, diet quality, race and socioeconomic factors affect school performance. Promotion of healthy eating and active living with consideration of racial and socioeconomic differences may therefore be beneficial both to prevent chronic diseases and to improve school performance and cognitive development.

The importance of school performance for self-esteem is better established (13,14). Good school performance makes students feel better and more confident, which in turn may help them set higher aspirations, foster the confidence to deal with difficult problems and provide the willingness or intrinsic motivation to persist in the face of initial failure (13). The findings of the present study are in agreement with these earlier studies.

The strengths of the present study are that it is a large population-based study with objective measures of height and weight and included results from standardized literacy test. However, assessment tools for self-esteem are based on self-report and therefore prone to error. In addition, selection bias may have occurred as participation was voluntary and required parental consent, urging caution with generalizing the study observation to the population at large. We applied SEM and revealed that body weight and school performance affect self-esteem. Whereas some researchers who apply SEM would argue that these associations should be interpreted as causal, epidemiologists would object such an interpretation as the observations were cross-sectional (41–44). Acknowledging these differences in point of view, we recommend longitudinal studies to confirm the present findings. For example, the present childhood obesity epidemic has urged many jurisdictions to prepare for, or have already implemented, school-based promotion of healthy eating and active living. These initiatives also provide a unique opportunity to prospectively evaluate the importance of healthy eating, active living and body weight for self-esteem and for school performance.

In brief, we have shown that obesity is an important determinant for self-esteem in 10- to 11-year-old children. Obesity did not seem to have an independent effect on school performance. However, the established determinants of obesity, poor diet and inactivity, were found to affect both self-esteem and school performance. In other words, promotion of healthy eating and active living is not only important for the prevention of overweight and related chronic diseases; it may also improve mental health and cognitive development.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References

We thank all of the students, parents and schools for their participation. Also, we thank the research assistants and public health staff who assisted in the data collection, Nova Scotia Department of Education and Angela Fitzgerald, Nova Scotia Health Promotion and Protection. This research was funded by the Canadian Population Health Initiative and through a Canada Research Chair in Population Health and Alberta Heritage Foundation for Medical Research Scholarship to Dr Paul J. Veugelers and through an Alberta Heritage Foundation for Medicine Research traineeship award to Dr Fangfang Wang.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Research methods and procedures
  5. Results
  6. Discussion
  7. Conflict of Interest Statement
  8. Acknowledgements
  9. References